1. Field of the Invention
The invention relates to the art of image rendering. It finds application where color images are rendered with a single colorant. For example, the invention finds application in repro-graphic machines where a color image is scanned and then transformed for rendering so that single colorant (black and white) copies of the color image can be made. The invention finds further application in general-purpose computing devices such as personal computers and business graphics authoring devices. In the latter devices, color images such as bar and pie charts are created in color and displayed on a computer display and then are printed on black and white printers through the use of the invention. All these devices and devices like them are referred to herein as image processors.
2. Description of Related Art
Communicating concepts and ideas to others can be difficult. One method often used to communicate ideas to a group of people is to make a visual presentation. In a visual presentation, images such as charts, graphs and photographs are often displayed before the audience while the speaker explains and describes the significance of the images. Alternatively, the images can act as summaries of an associated speech. Typically, the images are presented in color. Color often adds to the impact and clarity of an image. For example, a pie chart or a bar graph is easier to read if its various sections are presented in different colors.
Copies of visual presentation material are often distributed to the audience members. The distributed materials serve to document the presentation. The distributed material can help audience members follow the presentation and can serve as study aids and/or reference material.
Unfortunately, it can be impractical or prohibitively expensive to distribute a large number of color copies of presentation material. Furthermore, in some cases, color reproduction equipment is not readily available. In these cases, the color images are often reproduced in black and white.
Creating black and white versions of color images can be problematic. Typically, a great deal of information is lost in the conversion to black and white. For example, typical, color image authoring devices can produce over sixteen million different colors, while typical black and white rendering devices can only produce two hundred fifty six shades of gray. Obviously, a great number of colors must be mapped to each level of gray. Therefore, portions of a color image that are quite obviously different colors can appear to be the same color when the image is rendered in black and white. When the image portions in question are, for example, different sections of a pie chart, this loss of information can render the pie chart useless.
Attempts have been made to alleviate this problem by using texturing to increase the number of ways colors can be represented in a black and white image. Typically, under these strategies, the conversion to black and white is accomplished by dividing colors into a finite number of bins and assigning a different halftone pattern to each of the bins. This approach can preserve more information from the color image. However, this approach can also lead to abrupt transitions in the black and white image. Where colors in the original image smoothly blend from one color to another, the blend in color can cross a bin boundary, resulting in a sudden shift in a halftone pattern or level. This problem is particularly pronounced when colors are near neutral or gray. This situation can be further aggravated by the presence of noise in the image. For example, a subtle jitter or shift in the color in a photograph of a person's face can be transformed into dramatic changes in halftone patterns if the jitter or shift is across one or more bin boundary.
Therefore, a method for rendering a color image in black and white is desired that preserves more of the information present in a color image. Furthermore a method is desired that accurately portrays smooth or subtle transitions in color as equally smooth or subtle transitions in texture.